Various wheel cover retention systems are known for retaining a plastic wheel cover to the wheel of a vehicle. One type employs a plurality of decorative caps which are carried by the wheel cover and thread into engagement with externally threaded lug nuts of the wheel to secure the wheel cover in place on the wheel. U.S. Pat. No. 4,998,780 ('780) discloses such a wheel cover retention system. The caps are formed with retaining legs that extend from an open end of the caps and terminate at barbed ends which extend through openings in the wheel cover to lock the caps on the wheel cover. The caps are threaded onto the wheel's lug nuts bringing a mounting shoulder of the cap members to bear against recessed flanges of the cap member, forcing the center of the cover inwardly and urging the perimeter of the wheel cover against the rim of the wheel. In effort to prevent the caps from unthreading from the lug nuts during use, a series of interlocking ribs is provided at the interface between the engaging surfaces of the caps and cover. In spite of the ribs, the caps may loosen if not fully tightened to begin with or conditions change lessening the compression force at the engaging cap and cover member surfaces, such as temperature changes or excessive vibrations.